What is the best way to spray my empty pens?

MS DryCare is the best answer!

You probably use dry powder or lime to keep your empty pens dry. Smart, especially if you realize that not all products are the same. For example, a dry powder does not have to be the cheapest to allow one to save. Take MS DryCare, one of the few powders that absorb more than 200% (!) of its weight in moisture. As a result, you need about five times less than for example lime. Count your profit. The pleasant eucalyptus scent also makes MS DryCare extra beneficial to the respiratory tract.

MS DryCare Plus

MS DryCare Plus is a hygiene powder, just as the normal MS DryCare, but with a little extra. MS DryCare Plus is foremost a drying agent, but also acts as an active disinfectant. In tests done by Abbott Analytical it clearly showed that MS DryCare Plus can be used against the most common diseases present at a farm.